1. Field of the Disclosure
This document relates to phosphorescent compounds and organic light emitting devices using the same, and more particularly, to organic light emitting diode devices using phosphorescent compounds having high triplet energy as the host of an emission layer.
2. Discussion of the Related Art
With the development of multimedia, flat panel displays (FDPs) are becoming more and more important. Accordingly, a variety of flat panel displays such as liquid crystal display (LCDs), plasma display panels (PDPs), field emission displays (FEDs), organic light emitting diode devices, and the like are put to practical use.
Among them, an organic light emitting diode device can be formed on a flexible transparent substrate, such as plastic, can be driven at a lower voltage (below 10V) than a plasma display panel or inorganic light emitting diode display, has relatively low power consumption, and has a superior color sense. Further, the organic light emitting diode device can represent three colors of green, blue and red, and thus is drawing a great deal of attention as a next-generation full-color display device.
The organic light emitting diode device can be formed by sequentially laminating an anode, a hole injection layer, a hole transport layer, an emission layer, and electron transport layer, an electron injection layer, and a cathode. For a light-emitting material, holes injected from the anode are recombined with electrons injected from the cathode to form excitons. Singlet excitons and triplet excitons are involved in the fluorescence and phosphorescence processes, respectively. In recent years, there is a growing trend that phosphorescent materials are replacing fluorescent materials. For a fluorescent material, singlets representing only 25% of excitons formed in the emission layer are used to produce light, and triplets representing 75% of the excitons are mostly lost and transformed into heat. On the other hand, phosphorescent material has a light emission mechanism for converting both singlet and triplet excitons into light.
A light emitting process of a phosphorescent material will be discussed briefly. Holes injected from the anode and electrons injected from the cathode meet in a host material of the emission layer. Thought a hole and an electron may be paired in a dopant in some cases, generally, a large amount of holes and electrons meet in the host due to high concentration of the host. At this point, energy transfer takes place from the singlet excitons formed in the host to the singlets or triplets of the dopant, and energy transfers takes place from the triplet excitons to the triplets of the dopant.
Since the excitons transferred to the singlets of the dopant are transferred to the triplets of the dopant by intersystem crossing, the first destination of all the excitons is a triplet level of the dopant. The thus-formed excitons are transferred to the ground state, and emit light. If the triplet energy of the hole transport layer or electron transport layer adjacent to the front and back of the emission layer is less than the triplet energy of the dopant, backward energy transfer takes place from the dopant or host to these layers, and this leads to an abrupt decrease in efficiency. Accordingly, the triplet energy of the hole/electron transport layers, as well as the host material of the emission layer, plays a very important role in phosphorescent devices.
For efficient energy transfer from the host to the dopant, the triplet energy of the host needs to be greater than the triplet energy of the dopant. For example, CBP, which is recently widely used, has a triplet energy of 2.6 eV. Therefore, in the case of a well-known Firpic phosphorescent dopant, backward energy transfer (thermal absorption) occurs from the host to the dopant, thus causing a decrease in efficiency. Moreover, high glass transfer temperature is required because low glass transfer temperature of materials affects thermal stability and weakens the properties of the device. Accordingly, there is an urgent need for the development of novel phosphorescent materials with high glass transfer temperature.